High Voltage Direct Current (HVDC) electric power systems can provide an economic and efficient alternative to High Voltage Alternating Current (HVAC) power systems, particularly when transporting power over long distances (>50 km), where HVDC generally has lower losses and can provide increased transmission capacity.
HVDC systems may also be used to facilitate the transfer of power between asynchronous networks and networks operating at different AC frequencies and/or to help stabilise AC networks.
One particular area of interest is the use of HVDC systems to transport power from remote renewable energy sources such as from offshore wind or hydro farms to the mainland.
One challenge with the use of DC systems over AC is breaking the current in the event of a power surge or fault. In an AC system the current goes through zero every half cycle and thus a mechanical breaker can be operated at or near a current zero to avoid any significant arcing as the breaker opens. However in a DC system there is no such break in current.
Mechanical breakers that are capable of effectively breaking DC currents are known, however they are often too slow for modern power systems, taking several tens of milliseconds. Breakers using semiconductor switches can overcome this problem, however semiconductor switches tend to have high conduction losses and the losses associated with a semiconductor breaker can be as high as 30% of the losses of a voltage source converter station.